There exist a class of coals which exhibit a property known as "caking." Such coals tend to soften and become somewhat sticky at relatively low temperatures. As these coals are heated in bed type gasifiers, having for example, fluidized, fixed or moving beds, there is a strong tendency for the coals to agglomerate and interfere with the orderly gasification process. Pretreating this class of coals with air or oxygen at a relatively low temperature, typically about 800.degree. F. overcomes this agglomeration tendency.
Pretreatment has been practiced at both low and high pressure. The low pressure operation is at essentially atmospheric pressure. The offgas produced thereby has a low heating value and is produced at pressures suitable only for use as fuel to boilers or furnaces. Low pressure pretreaters are generally large and require a relatively long holding time.
Contrastingly, high pressure pretreaters, operate at essentially gasification pressures and provide a more useful, a higher heat value, offgas. The offgas of these pretreaters can be combined and used with gasifier product fuel gas. Using air to pretreat limits such systems to the manufacture of the low heating value gas. There is no such limitation if oxygen is used for both gasification and pretreatment.
One presently conceived pretreater utilizes oxygen and enough steam to assure fluidization of the pretreater bed. The offgas from the pretreater is conducted to the bottom of the gasifier bed and there joins the produced fuel gas. For this system coal or other carbonaceous solid is fed from for example, a standpipe and valve, a screw conveyor, or other mechanical flow regulating device, directly to the top of the pretreater bed. The treated coal overflows into a standpipe which includes a solid flow control device, and is injected into the gasifier at the top of the bed. Temperature control is achieved by coil heat exchangers immersed in the pretreater bed. This system is more fully disclosed in U.S. Pat. No. 4,057,402 issued on Nov. 8, 1977 to J. G. Patel, and in the article entitled "Clean Fuel From Coal is the Goal of U-Gas Process" by J. G. Patel, The Oil and Gas Journal, Aug. 1, 1977.
Another pretreater conception uses oxygen for pretreatment and sufficient steam to control the pretreater temperature and to convey the coal into the pretreater bed. Precontacting the coal with oxygen reduces the holding time needed in the coal bed of the pretreater. Both offgas and treated coal overflow together into the gasifier bed. An example of this system can be seen in an article entited "Synthane Gasification Proves Responsive" by Robert Lewis in The Oil and Gas Journal, Aug. 1, 1977.
Problems and disadvantages associated with the above and other prior art devices include the following. First, most systems are comprised of several separate subsystems such a separate pretreaters and separate gasifiers. Consequently complex and costly conduits must be designed into the system to transfer offgases and coal between the subsystems.
Second, where the transfer of coal between the pretreaters and gasifiers is accomplished by the overflow of pretreated coal from the pretreater bed into a conduit, there is no mechanism for controlling the residence time of the coal in the pretreater bed by varying the bed level and thus, the system cannot be adjusted for optimum operations.
Third, many existing systems use cooling coils located in the pretreater to control the temperature of the pretreater. Such an arrangement introduces undesirable thermal inefficiencies into the system.
Fourth, some of the prior art systems do not provide for good mixing of newly introduced offgases and pretreated coal with the char in the gasifier.
Accordingly, an object of the present invention is to provide a unitary, integral system which incorporates a pretreater and a gasifier with a minimum of piping and conduits.
Another object of the present invention is to provide for the selective control of the coal bed level in the pretreater, hence the residence time of the coal in the pretreater, to optimally operate the pretreater and gasifier.
Yet another object is to efficiently cool the pretreater to control the temperature thereof.
Still another object is to provide for good mixing of introduced offgas and coal from the pretreater with the char in the gasifier.
Another object is to agglomerate ash in the gasifier section to efficiently remove ash therefrom.